Research

4578 KiB

Open AccessArticle

Investigation of the Frozen Bath Layer under Cold Anodes

by Donald Picard, Jayson Tessier, Guillaume Gauvin, Donald Ziegler, Houshang Alamdari and Mario Fafard

Metals 2017, 7(9), 374; https://doi.org/10.3390/met7090374 - 15 Sep 2017

Cited by 5 | Viewed by 3442

Abstract

Hall-Héroult cell stability is highly affected by anode changing operations. Upon the insertion of a cold anode in the cell, a layer of molten cryolite freezes under the anode. The thickness, microstructure, and chemical composition of this layer vary as a function of [...] Read more.

Hall-Héroult cell stability is highly affected by anode changing operations. Upon the insertion of a cold anode in the cell, a layer of molten cryolite freezes under the anode. The thickness, microstructure, and chemical composition of this layer vary as a function of time and its location in the cell. To better understand the evolution of the frozen layer, mandatory for the validation of numerical models, a measurement campaign was conducted on the anodes having a few hours of operation in the cell. The macrostructure of the selected frozen bath samples has been investigated using computed tomography while scanning electron microscope (SEM) has been used to qualify its microstructure. An energy-dispersive X-ray spectroscope (EDS) coupled to the SEM has revealed the chemical content. The results showed not only very different macrostructures between samples, but also significantly heterogeneous structure within the same sample. Nevertheless, for all samples, there is a clear distinction between the frozen cryolite and alumina/dusting phases, with the latter surrounding the cryolite matrix. Full article

(This article belongs to the Special Issue Selected Papers from the International Committee for Study of Bauxite, Alumina & Aluminium 2016)

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6961 KiB

Open AccessArticle

Electrical Resistivity Measurement of Carbon Anodes Using the Van der Pauw Method

by Geoffroy Rouget, Hicham Chaouki, Donald Picard, Donald Ziegler and Houshang Alamdari

Metals 2017, 7(9), 369; https://doi.org/10.3390/met7090369 - 13 Sep 2017

Cited by 1 | Viewed by 6154

Abstract

The electrical resistivity of carbon anodes is an important parameter in the overall efficiency of the aluminum smelting process. The aim of this work is to explore the Van der Pauw (VdP) method as an alternative technique to the standard method, which is [...] Read more.

The electrical resistivity of carbon anodes is an important parameter in the overall efficiency of the aluminum smelting process. The aim of this work is to explore the Van der Pauw (VdP) method as an alternative technique to the standard method, which is commonly used in the aluminum industry, in order to characterize the electrical resistivity of carbon anodes and to assess the accuracy of the method. For this purpose, a cylindrical core is extracted from the top of the anodes. The electrical resistivity of the core samples is measured according to the ISO 11713 standard method. This method consists of applying a 1 A current along the revolution axis of the sample, and then measuring the voltage drop on its side, along the same direction. Theoretically, this technique appears to be satisfying, but cracks in the sample that are generated either during the anode production or while coring the sample may induce high variations in the measured signal. The VdP method, as presented in 1958 by L.J. Van der Pauw, enables the electrical resistivity of any plain sample with an arbitrary shape and low thickness to be measured, even in the presence of cracks. In this work, measurements were performed using both the standard method and the Van der Pauw method, on both flawless and cracked samples. Results provided by the VdP method appeared to be more reliable and repeatable. Furthermore, numerical simulations using the finite element method (FEM) were performed in order to assess the effect of the presence of cracks and their thicknesses on the accuracy of the VdP method. Full article

(This article belongs to the Special Issue Selected Papers from the International Committee for Study of Bauxite, Alumina & Aluminium 2016)

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5262 KiB

Open AccessArticle

Inspection of Prebaked Carbon Anodes Using Multi-Spectral Acousto-Ultrasonic Excitation

by Moez Ben Boubaker, Donald Picard, Carl Duchesne, Jayson Tessier, Houshang Alamdari and Mario Fafard

Metals 2017, 7(8), 305; https://doi.org/10.3390/met7080305 - 08 Aug 2017

Cited by 4 | Viewed by 4152

Abstract

Reduction cell operation in primary aluminum production is strongly influenced by the properties of baked anodes. Producing consistent anode quality is more challenging nowadays due to the increasing variability of raw materials. Taking timely corrective actions to attenuate the impact of raw material [...] Read more.

Reduction cell operation in primary aluminum production is strongly influenced by the properties of baked anodes. Producing consistent anode quality is more challenging nowadays due to the increasing variability of raw materials. Taking timely corrective actions to attenuate the impact of raw material fluctuations on anode quality is also difficult based on the core sampling and characterization scheme currently used by most anode manufacturers, because it is applied on a very small proportion of the anode production (about 1%), and long-time delays are required for lab characterization. The objective of this work is to develop rapid and non-destructive methods for the inspection of baked anodes. Previous work has established that sequential excitation of smaller parts collected from an industrial sized anode using acousto-ultrasonic signals at different frequencies allowed detecting and discriminating anode defects (pores and cracks). This was validated qualitatively using X-ray computed tomography. This work improves the method by using frequency-modulated excitation and building quantitative relationships between the acousto-ultrasonic signals and defects extracted from tomography images using Wavelet Transforms and Partial Least Squares (PLS) regression. The new excitation approach was found to provide similar or better inspection performance compared with sequential excitation, while requiring a shorter cycle time. Full article

(This article belongs to the Special Issue Selected Papers from the International Committee for Study of Bauxite, Alumina & Aluminium 2016)

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3040 KiB

Open AccessFeature PaperArticle

Discrete Element Method Investigation of Bulk Density and Electrical Resistivity of Calcined Coke Mixes

by Behzad Majidi, Geoffroy Rouget, Mario Fafard, Donald Ziegler and Houshang Alamdari

Metals 2017, 7(5), 154; https://doi.org/10.3390/met7050154 - 26 Apr 2017

Cited by 5 | Viewed by 4345

Abstract

Packing density and electrical resistivity of particles assemblies are important factors for a variety of applications of granular materials. In the present work, a three-dimensional imaging technique is coupled with the discrete element method (DEM) to model anode grade calcined coke particles. Three-dimensional [...] Read more.

Packing density and electrical resistivity of particles assemblies are important factors for a variety of applications of granular materials. In the present work, a three-dimensional imaging technique is coupled with the discrete element method (DEM) to model anode grade calcined coke particles. Three-dimensional DEM models of samples with different size distribution of particles were studied to obtain the inter-particle contact information. As the content of fine particles increased, a higher inter-particle contact density and smaller average contact radius was observed in the samples. Confronting the DEM data and experimental measurements of electrical resistivity showed the simultaneous effects of packing density and contact density. Samples with higher contact density and smaller contact radius in general held high electrical resistivities. However, if increasing the contact density does not modify contacts between large particles, this will have a positive effect on packing density, so a lower electrical resistivity was obtained. Full article

(This article belongs to the Special Issue Selected Papers from the International Committee for Study of Bauxite, Alumina & Aluminium 2016)

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4973 KiB

Open AccessArticle

A Non-Destructive Technique for the On-Line Quality Control of Green and Baked Anodes †

by Abderrahmane Benzaoui, Duygu Kocaefe, Dipankar Bhattacharyay and Yasar Kocaefe

Metals 2017, 7(4), 128; https://doi.org/10.3390/met7040128 - 06 Apr 2017

Cited by 6 | Viewed by 3807

Abstract

Carbon anodes play an important role in the electrolytic production of aluminum. They have a significant economic and environmental impact. Carbon anodes are made of dry aggregates, composed of petroleum coke, recycled rejects, and butts, bound by coal tar pitch. Due to several [...] Read more.

Carbon anodes play an important role in the electrolytic production of aluminum. They have a significant economic and environmental impact. Carbon anodes are made of dry aggregates, composed of petroleum coke, recycled rejects, and butts, bound by coal tar pitch. Due to several factors, defects (cracks/pores) appear in anodes during the fabrication process, affecting their quality. It is thus essential to control the quality of anodes before their use in the electrolysis cell. Current practice for the quality evaluation (visual inspection, core analysis) gives limited information. As an alternative to this practice, electrical resistivity measurements can be used. Electrical resistivity is one of the key indicators for anode quality and its homogeneity. A simple and non-destructive method has been developed for the specific electrical resistivity measurement of anodes (SERMA) for on-line control of anode quality. Various tests have been carried out at both lab scale and industrial scale. In this study, the electrical resistivity distributions in the lab-scale anodes were measured and compared with those of the tomography analysis. The method is able to detect defective anodes even before the baking process. Full article

(This article belongs to the Special Issue Selected Papers from the International Committee for Study of Bauxite, Alumina & Aluminium 2016)

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2409 KiB

Open AccessArticle

Evolution of Anode Porosity under Air Oxidation: The Unveiling of the Active Pore Size

by Francois Chevarin, Ramzi Ishak, Donald Ziegler, Mario Fafard and Houshang Alamdari

Metals 2017, 7(3), 101; https://doi.org/10.3390/met7030101 - 18 Mar 2017

Cited by 3 | Viewed by 5502

Abstract

The carbon anode, used in aluminum electrolysis (Hall–Héroult process), is over-consumed by air oxidation and carboxy-reaction (with CO₂). Several anode features may affect this over-consumption, such as impurity content, graphitization level and anode porosity features (e.g., porosity volume fraction or pore [...] Read more.

The carbon anode, used in aluminum electrolysis (Hall–Héroult process), is over-consumed by air oxidation and carboxy-reaction (with CO₂). Several anode features may affect this over-consumption, such as impurity content, graphitization level and anode porosity features (e.g., porosity volume fraction or pore size distribution). The two first parameters are basically related to the quality of raw materials and coke calcination conditions. Anode porosity is, however, greatly affected by anode manufacturing conditions, and is possible to be modified, to some extent, by adjusting the anode recipe and the processing parameters. This work aims to investigate the effect of anode porosity on its air reactivity. Baked anode samples were prepared in laboratory scale and then crushed into powder form (−4760 + 4000 µm). The recipe for anode preparation was similar to a typical industrial recipe, except that in the lab scale no butt particles were used in the recipe. Anode particles were then gasified at six different conversion levels (0, 5, 15, 25, 35 and 50 wt %) under air at 525 °C. The porosity was characterized in several pore size ranges, measured by nitrogen adsorption and mercury intrusion (0.0014–0.020, 0.002–0.025, 0.025–0.100, 0.1–40.0 and superior at 40 µm). The volume variation of each pore range, as a function of carbon conversion, was assessed and used to determine the size of the most active pores for air oxidation. The most active pore size was found to be the pores inferior at 40 µm before 15 wt % of gasification and pores superior at 40 µm between 15 and 50 wt % of carbon conversion. Limitation of pore size range could be used as an additional guideline, along with other targets such as high homogeneity and density, to set the optimum anode manufacturing parameters. Full article

(This article belongs to the Special Issue Selected Papers from the International Committee for Study of Bauxite, Alumina & Aluminium 2016)

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1960 KiB

Open AccessArticle

Effects of Charcoal Addition on the Properties of Carbon Anodes

by Asem Hussein, Mario Fafard, Donald Ziegler and Houshang Alamdari

Metals 2017, 7(3), 98; https://doi.org/10.3390/met7030098 - 16 Mar 2017

Cited by 20 | Viewed by 5128

Abstract

Wood charcoal is an attractive alternative to petroleum coke in production of carbon anodes for the aluminum smelting process. Calcined petroleum coke is the major component in the anode recipe and its consumption results in a direct greenhouse gas (GHG) footprint for the [...] Read more.

Wood charcoal is an attractive alternative to petroleum coke in production of carbon anodes for the aluminum smelting process. Calcined petroleum coke is the major component in the anode recipe and its consumption results in a direct greenhouse gas (GHG) footprint for the industry. Charcoal, on the other hand, is considered as a green and abundant source of sulfur-free carbon. However, its amorphous carbon structure and high contents of alkali and alkaline earth metals (e.g., Na and Ca) make charcoal highly reactive to air and CO₂. Acid washing and heat treatment were employed in order to reduce the reactivity of charcoal. The pre-treated charcoal was used to substitute up to 10% of coke in the anode recipe in an attempt to investigate the effect of this substitution on final anode properties. The results showed deterioration in the anode properties by increasing the charcoal content. However, by adjusting the anode recipe, this negative effect can be considerably mitigated. Increasing the pitch content was found to be helpful to improve the physical properties of the anodes containing charcoal. Full article

(This article belongs to the Special Issue Selected Papers from the International Committee for Study of Bauxite, Alumina & Aluminium 2016)

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2381 KiB

Open AccessArticle

Impact of the Solidification Rate on the Chemical Composition of Frozen Cryolite Bath

by Sándor Poncsák, László I. Kiss, Sébastien Guérard and Jean-François Bilodeau

Metals 2017, 7(3), 97; https://doi.org/10.3390/met7030097 - 16 Mar 2017

Cited by 5 | Viewed by 4806

Abstract

Solidification of cryolite (Na3AlF6)-based bath takes place at different rates along the sideledge, and around alumina rafts and new anodes. The solidification rate has a significant impact on the structure and the chemical composition that determine the thermal conductivity and thus the thickness [...] Read more.

Solidification of cryolite (Na3AlF6)-based bath takes place at different rates along the sideledge, and around alumina rafts and new anodes. The solidification rate has a significant impact on the structure and the chemical composition that determine the thermal conductivity and thus the thickness of sideledge, or the duration of the existence of the temporary frozen bath layers in other cases. Unfortunately, samples that can be collected in industrial cells are formed under unknown, spatially and temporally varying conditions. For this reason, frozen bath samples were created under different heat flux conditions in a well-controlled laboratory environment using the so-called cold finger technique. The samples were analyzed by X-ray Diffractometer (XRD) and Scanning Electron Microscope (SEM) in Back Scattering (BS) mode in order to obtain spatial distribution of chemical composition. Results were correlated with structural analysis. XRD confirmed our earlier hypothesis of recrystallization of cryolite to chiolite under medium heat flux regime. Lower α-alumina, and higher γ-alumina content in the samples obtained with very high heating rate suggest that fast cooling reduces α–γ conversion. In accordance with the expectation, SEM-BS revealed significant variation of the Na/Al ratio in the transient sample. Full article

(This article belongs to the Special Issue Selected Papers from the International Committee for Study of Bauxite, Alumina & Aluminium 2016)

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4306 KiB

Open AccessArticle

Application of Boron Oxide as a Protective Surface Treatment to Decrease the Air Reactivity of Carbon Anodes

by Ramzi Ishak, Donald Picard, Gaétan Laroche, Donald P. Ziegler and Houshang Alamdari

Metals 2017, 7(3), 79; https://doi.org/10.3390/met7030079 - 03 Mar 2017

Cited by 5 | Viewed by 5664

Abstract

The oxidation of a carbon anode with air and CO₂ occurs during the electrolysis of alumina in Hall-Héroult cells, resulting in a significant overconsumption of carbon and dusting. Boron is well known to decrease the rate of this reaction for graphite. In [...] Read more.

The oxidation of a carbon anode with air and CO₂ occurs during the electrolysis of alumina in Hall-Héroult cells, resulting in a significant overconsumption of carbon and dusting. Boron is well known to decrease the rate of this reaction for graphite. In this work, the application of boron oxide has been investigated to evaluate its inhibition effect on the air oxidation reaction, and to provide an effective protection for anodes. Different methods of impregnation coating have been explored. Impregnated anode samples were gasified under air at 525 °C according to the standard measurement methods. X-ray tomography was used to obtain the microstructural information of the samples before and after air-burning tests. The impregnated samples showed a very low oxidation reaction rate and dust generation. Full article

(This article belongs to the Special Issue Selected Papers from the International Committee for Study of Bauxite, Alumina & Aluminium 2016)

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2731 KiB

Open AccessArticle

The Effect of Varying Mixing Temperatures and Baking Level on the Quality of Pilot Scale Anodes—A Factorial Design Analysis

by Camilla Sommerseth, Rebecca Jayne Thorne, Arne Petter Ratvik, Espen Sandnes, Hogne Linga, Lorentz Petter Lossius and Ann Mari Svensson

Metals 2017, 7(3), 74; https://doi.org/10.3390/met7030074 - 25 Feb 2017

Cited by 5 | Viewed by 4294

Abstract

Identifying optimum anode baking level and mixing temperature are important when producing high quality anodes. The effect of varying mixing temperature and baking level were investigated in terms of the resulting apparent anode density, specific electrical resistivity (SER), air permeability, coefficient of thermal [...] Read more.

Identifying optimum anode baking level and mixing temperature are important when producing high quality anodes. The effect of varying mixing temperature and baking level were investigated in terms of the resulting apparent anode density, specific electrical resistivity (SER), air permeability, coefficient of thermal expansion (CTE), air reactivity, and CO₂ reactivity. Six pilot-scale anodes were prepared at Hydro Aluminium using a single source petroleum coke and <2 mm coke fractions. A coal tar pitch was used with Mettler softening point of 119.1 °C. The aggregate was mixed at 150 °C or 210 °C and baked to a low, medium, or high baking level. A 2² full-factorial design analysis was performed to determine the response of the analyzed properties to the applied mixing and baking temperature. Apparent density, SER, and air permeability were found to be highly dependent on mixing temperature. Apparent density and SER were also slightly affected by baking level. CTE was found to be independent of both baking level and mixing temperature. Air reactivity was found to be mainly dependent on baking level, while CO₂ reactivity was dependent on both mixing temperature and baking level. Full article

(This article belongs to the Special Issue Selected Papers from the International Committee for Study of Bauxite, Alumina & Aluminium 2016)

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